The invention relates to an integrated antenna for transmitting and/or receiving terahertz radiation, and to a transmitter and/or receiver device for terahertz radiation including such an antenna, and also to a method of fabricating said antenna and said device.
The term “terahertz” radiation covers electromagnetic radiation at a frequency lying in the range 100 gigahertz (GHz) to 3 terahertz (THz) approximately (1 THz=1012 hertz (Hz)=1000 GHz). In a more restrictive sense, terahertz radiation is taken as being synonymous with submillimetric radiation, i.e. radiation having a wavelength lying in the range 100 micrometers (μm) to 1 millimeters (mm) approximately (approximate frequency lying in the range 300 GHz to 3 THz).
Terahertz radiation constitutes a spectral range intermediate between microwaves and infrared. Applications thereof were for a long time marginal or non-existent, but they are now in full expansion. Amongst those applications, the most important are spectroscopy and imaging for detecting pollution, for non-destructive inspection, and for medical diagnosis. In general, these applications make use of terahertz radiation propagating in empty space: it is therefore necessary to provide transmitters and receivers with antennas.
The antennas generally used at terahertz frequencies are planar antennas, typically of the dipole type (Hertz dipole), made monolithically on the substrate having the active components used for generating or detecting the radiation integrated thereon and including a hemispherical lens of silicon placed on the rear face of the substrate. Such antennas are described in particular in the document U.S. Pat. No. 5,789,750 and in the article by G. M. Rebeiz, “Millimeter-wave and terahertz integrated circuit antennas”, Proceedings of the IEEE, Vol. 80, No. 11, p. 1748 (1992).
Those devices present numerous drawbacks.
Firstly, making the lens out of silicon and positioning it on the substrate with accuracy of micrometer order relative to the antenna is very difficult and expensive. Secondly, the use of a coupling lens is essential in order to prevent the radiation that is emitted mainly into the semiconductor substrate becoming trapped therein. In spite of the presence of such a lens, only about 21% of the power emitted by a planar type antenna is actually radiated into empty space, with the remainder being trapped and absorbed by the substrate.
Furthermore, the planar antenna in the most widespread use, a Hertz dipole, presents poor efficiency and, worse, its efficiency depends strongly on frequency. In spite of the fact that such antennas are not genuinely “broadband” antennas, they have nevertheless made it possible to obtain the best results so far in pulse terahertz spectroscopy. However their poor efficiency makes them unsuitable for use under continuous conditions, where the powers involved are very small.
Document U.S. Pat. No. 4,855,749 describes a planar antenna of the Vivaldi type made on a silicon substrate, operating in the terahertz range and not requiring a coupling lens. The results obtained with the help of such an antenna are not entirely satisfactory, in particular from the points of view energy efficiency and large dispersion of pulses.
Document US 2006/0152412 describes a planar antenna in the form of a logarithmic spiral. Such an antenna presents relatively good efficiency and is a broadband antenna, but it is highly dispersive. Consequently, it is not suitable for use under pulse conditions.
The article by V. Lubecke et al., “Micromachining of terahertz applications”, IEEE Trans. on Microwave Theory and Tech., Vol. 46, No. 11, p. 1821 (1998) relates to integrated antennas for terahertz systems made with the help of microtechnology techniques and overcoming the limitations of planar structures. Nevertheless, the solutions provided by those methods are not satisfactory because of the cost and the complexity of the fabrication methods and because of the fragility of the resulting structures. In addition, the techniques used (deep etching, dielectric membrane, etc.) are appropriate essentially for silicon, a material that is not particularly suitable for terahertz applications.